A Columbiner is a person who expresses great interest in the Columbine Massacre.
The Columbiner fandom is sometimes known as the "Cult of Columbine". Most Columbiners simply have an academic interest in the shooters' motives and the happenings during the shooting, and do not condemn the shooters' actions whatsoever. There are a few who more or less can align with the perpetrators' problems, such as depression and bullying. Some express romantic interest in the shooters, such as Sol Pais. Finally, there are those who aspire to do similar things themselves, such as Lindsay Souvannarath, who planned the foiled Halifax Mall Shooting, and Pekka-Eric Auvinen, the perpetrator of the Jokela School Shooting.
The amount of copycats and followers the Columbine shooters have accumulated is largely due to the amount of media coverage on the shooters. #NoNotoriety is a movement which encourages news sites to leave the names and lives of mass murderers undisclosed to prevent such an effect from repeating itself in the future. The official #NoNotoriety site can be found here.